Eciton burchellii

Appearance

Like most ant species, "Eciton burchellii" is polymorphic, meaning that features amongst smaller groups within the colony vary in size: a colony contains workers ranging from 3 mm to 12 mm, with each specific "caste" suited to specialized tasks. At least four castes of workers exist in its social system. Like other species of "Eciton", "Eciton burchellii" features a highly modified soldier caste bearing long, pointed, characteristically falcate mandibles. These features are much larger than on the "porter caste", the caste directly below that of the major "soldiers". Their long legs and elongated body lend them a spider-like appearance. Color varies from deep golden to dark brown. Workers possess single-faceted compound eyes, double-segmented waists, a well-developed sting, and specialized tarsal hooks on their feet with which they cling to one another to form bridges and bivouacs.

Naming

A total of 557 different species of animal have been found to associate in some way with "E. burchellii", the greatest number known for any individual species. An even larger number of species have been recorded living with the ants, but have not yet been identified. Although some of the associations are likely to be opportunistic, around 300 of the species are thought to be reliant on "E. burchellii" in some form to survive. The species is commonly attended by "ant following" birds, such as antbirds and woodcreepers. Insects and other arthropods attempting to escape from "E. burchellii" are flushed into the attending flocks, and a number of species have evolved behavior to obtain most of their food by following swarms. Butterflies are then attracted to the droppings of the birds following the ants, which they drink from. "Stylogaster" and "Calodexia" flies are abundant around raiding parties, laying eggs on fleeing cockroaches; of the cockroaches that escape the ants, 50–90% are parasitised by the flies. Beetles that mimic the ants can be found in the bivouac and in columns.

Many mites live in the bivouacs and ant columns. On Barro Colorado Island, Panama, 5% of the 3156 worker ants examined had mites on them, with the Scutacaridae and Pygmephoridae families being the most abundant. The mites are mainly thought to be harmless to the ants, being symbionts rather than parasites. They most likely were present to exploit the hosts for mechanical transportation or to use their waste deposits.

Behavior

Social animals need an alarm system to alert others to defend against potential threats or to recruit others to attack prey. In "Eciton burchellii", along with other large-colony ant species, the alarm pheromone is produced in mandible glands. This is evolutionarily advantageous because the mandible has a large surface area for pheromone's evaporation, the pheromone is released whenever the mandible is opened for biting, and the pheromone is rapidly released when the ant's head is crushed. The specific pheromone used by the "Eciton burchellii" species is 4-methyl-3-heptanone, which produces an intense, but short-lived, behavioral response by others in the colony.

Defense

Unlike other Hymenoptera species, ants cannot actively thermoregulate through processes such as evaporation, fanning, or incubation. Therefore, they must rely on the location and architecture of their nests in order to regulate their temperature. For species of ants that migrate frequently, such as the "Eciton burchellii", the location of the nest may be the most important thermoregulation tool. However, "Eciton burchellii" does not construct a physical nest. Instead, it builds a living nest out of the individual colony members. Thermoregulation within these bivouacs is accomplished through the opening or sealing of airways. The colony members can also manipulate the bivouac to avoid rainfall or direct sunlight. Bivouacs are often found in hollow logs, animal burrows in the ground, and hanging in trees. During each nomadic phase, a new nesting site must be found. On average, it takes a colony six to eight hours to move their bivouac. This occurs primarily during the night.